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Growth inhibitor found in Centaurea spp. Fletcher, Ronald Austin


The knapweeds were introduced into North America in the nineteenth century. They have now established themselves and are spreading rapidly. In British Columbia’s dry interior they grow in dense infestations, tending to exclude as well as out-compete other plants. Because of this characteristic they now present a serious problem. It has been suspected that toxic substances produced by the knapweeds may be partially responsible for enabling them to establish themselves at the expense of other plants. The object of this study, therefore, was to establish the presence and location of inhibitory substances in three species of knapweed: Centaurea repens, Centaurea diffusa, and Centaurea maculosa; and if possible to identify the chemical structure of the compounds responsible. In order to determine whether inhibitors might be present, tomatoes were grown in both infested and non infested soil. Also, dried leaf powder from the knapweed was added to greenhouse soil to observe the effects it might have on the growth of tomatoes. Leaf, stem, root and seed of the knapweed were extracted separately with different solvents: ether, water and ethanol. Germination tests, using lettuce, barley and cress as test crops, were used as methods of bioassay. In order to isolate the inhibitory compound, ascending paper chromatography was employed using the aqueous phase of n-Butanol:acetic acid:water (4:1:5) as the solvent. The inhibitor was located on the chromatographic paper by means of germination tests. Chromogenic sprays and comparisons of Rf values with different solvents were made in order to identify the inhibitor. To further characterize the compound an ultraviolet spectrum was carried out. The findings of the soil studies indicated that knapweed infested soil, and soil containing leaf powder of knapweed, were inhibitory to the growth of tomatoes. The inhibitor was found to be present in all the three species studied, and was located mainly in the leaves. It proved to be both ether and water soluble, and non-specific, being inhibitory to all the crops tested. The inhibitor was found as a pale yellow fluorescent band at an Rf of 0.9. Chromogenic sprays indicated the inhibitor to be an indole derivative. This was confirmed by the ultraviolet absorption spectrum.

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